I currently have two projects running: SETI@Home and Preditor@Home . Is there any way to pause the work on one of the projects or to change the resource shares once I have attached the project? If I use the "detatch" or "reset" options will I loose any credit I have earned for the project?

No, you can't pause the work on one of the projects. The only way to "pause" a project would be to detach.

No, detaching or resetting a project will not cause you to loose any credit you have earned, nor will it wipe any credits in pending. It will however, flush any WUs you have downloaded, thus causing a severe bog in credit granting for others waiting on those WUs as they need to be re-queued for sending to another host by the scheduler.